First, thank you for so kindly taking Moochi into your home. Many cats whose owners go into nursing homes, or worse, die, are sent to shelters.

You can imagine how distressed Moochi would be in a shelter!

You didn't include a time frame for these events. But sometimes it takes a long time for some cats to adjust. It is very fortunate that your animals are friendly and not harassing him.

He was taken from the only home he knew and is in a new, and to him, frightening and maybe threatening environment (he may perceive the other animal as a threat.)

Please be patient. Let him stay in the basement for now, with access to the rest of the house. Try to keep the other animals out of his space. Let him find one place in his new home that he can feel confident and safe. Go be with him, and ignore him if your attention aggravates his behavior. Sit and read a book, or take your laptop down there. Let him get used to your presence, and if he softens, respect his distance and talk to him.

Go slow. Be patient.

He's as distressed in his new environment as he'd be if placed in a shelter. But in your quiet home he'll eventually adjust where in a shelter...who knows what would happen.

He's also grieving for his companion-your FIL was probably this cat's whole life. Displaced aggression could be from the sudden change in his circumstances plus the meeting with your house pets. He doesn't know what to do about any of this and needs a while to sort it out.

Is there any chance you can shut yourself in with him for an hour or so once or twice daily? Several visits per day are more unsettling than a few long sits. Take a book to while away the time and just sit quietly. He'll approach when he's ready.

I've read many testimonials to the efficacy of Feliway diffusers. They're far from cheap but all the testimony can't be discounted. Feliway is a synthetic feline pheromone like that left behind when cats cheek-rub stuff. It's supposed to be a signal that all is well. Your other cat should also respond well to the Feliway. You can ask the vet about a course of antidepressants. This doesn't have to be a forever thing but it will take the edge off of Moochie's grief and despair while you teach him that he's loved and welcome in your home.

I've introduced many cats over the years, and find that it takes as long as a year for the ruffled feathers to settle down. It's a long frustrating experience but under your guidance all the house pets can benefit.

Good advice already. Have you by chance seen that television show "My Cat From Hell"? He deals with a lot of fearful cat situations and helps people help their cats become more confident in their environment. There is a website devoted to the show that has some good advice, and you can watch video clips from the show there as well.